A switch mode power converter (also referred to as a “power converter”) is a power supply or power processing circuit that converts an input voltage waveform into a specified output voltage waveform. Controllers associated with the power converters manage an operation thereof by controlling the conduction periods of switches employed therein. Generally, the controllers are coupled between an input and output of the power converter in a feedback loop configuration (also referred to as a “control loop” or “closed control loop”).
Typically, the controller measures an output characteristic (e.g., an output voltage) of the power converter and based thereon modifies a duty cycle of the switches of the power converter. The duty cycle is a ratio represented by a conduction period of a switch to a switching period thereof. Thus, if a switch conducts for half of the switching period, the duty cycle for the switch would be 0.5 (or 50 percent). Additionally, as the needs for systems such as a microprocessor powered by the power converter dynamically change (e.g., as a computational load on the microprocessor changes), the controller should be configured to dynamically increase or decrease the duty cycle of the switches therein to maintain the output characteristic at a desired value.
For the most part, controllers associated with the power converters have primarily been composed of interconnected analog circuits. Analog circuitry, however, is undesirable for numerous reasons as set forth below. First, analog controllers can require a multitude of interconnected discrete components to obtain a desired functionality of a single controller, which translates into large inventory costs, and more complicated and expensive manufacturing processes. The analog controllers also tend to take up a fairly extensive footprint of the power converters to accommodate the numerous discrete components therein. With a trend toward smaller power converters that parallels an increased level of integration with the loads powered thereby, employing a larger footprint for the controller necessitated by the numerous discrete components that are not easily reduced in size by circuit integration is disadvantageous.
Additionally, analog hardware is generally fixed and inflexible thereby making modifications thereto very difficult, if not impossible, without a complete redesign of the controller. Analog circuitry is also subject to packaging and component defects, especially as the number of components increases, and analog circuitry tends to be sensitive to noise leading to further defects therein. As is evident from the foregoing, analog controllers suffer from several drawbacks that inhibit the effectiveness thereof. Despite the numerous limitations of analog circuitry, however, the analog controllers have been the controllers of choice for use with a majority of commercially available power converters today.
Looking at the alternatives to analog controllers, controllers employing digital circuitry have been successfully employed in lieu of analog controllers in power converters servicing lower speed or slower response applications (e.g., response times in a range of two to 100 milliseconds) such as uninterruptible power supplies, motor drives and three-phase rectifiers. The controllers employing digital circuitry that control power converters servicing such applications can generally execute the necessary commands in an allotted amount of time consistent with the respective application.
To date, however, controllers employing digital circuitry for use with power converters powering sensitive loads such as high performance microprocessors have not been viable alternatives to analog controllers due, in part, to a necessity for faster control loops and tighter budgetary constraints. A typical commercial AC-to-DC or DC-to-DC power converter demands response times of approximately 20 microseconds or less with closed control loop bandwidth requirements in a range of five to 100 kilohertz. The controllers for the power converters should also account for sampling within a switching cycle, which is typically two orders of magnitude faster than the lower speed applications mentioned above. The fast response times are presently attainable by controllers employing analog circuitry. Cost effective digital signal processors are generally not capable of meeting the wide bandwidth requirements in the control loops to meet the fast transient response times and high switching frequencies of the power converters.
Moreover, overcoming the aforementioned limitations may induce cost prohibitive processor designs, when compared to the cost of analog controllers. For instance, a 60 watt DC-to-DC power converter with analog control circuitry may cost around $35, whereas the cost of a digital signal processor alone for the power converter may cost around $15, which is a substantial cost for the control function by itself. A controller incorporating a high performance digital signal processor has, in the past, been too expensive for use with the power converters when compared to the lower cost analog controller.
It is recognized that a significant contributor to a complexity of digital processing and the related computational delays in the digital processing of control signals is a substitution of digital circuitry in the controller for corresponding analog processes. An example of such a substitution is sensing and converting an analog signal such as the output characteristic (e.g., the output voltage) with sufficient accuracy and speed into a digital format for use with the digital circuitry in the controller. The inverse process of a digital to analog conversion is less complex and, often times, can be performed expeditiously with an “R-2R” resistor ladder and an operational amplifier. The process of analog to digital conversion, however, generally uses techniques such as “successive approximation” that employ a significant amount of time for iterative processing. The process of analog to digital conversion may also use “flash conversions” that employ significant circuitry to perform the necessary tasks in an acceptable time period, or ramp generators and counters that take a significant amount of time to perform the conversion function. The aforementioned complexities all contribute either individually or in combination to the complexity of the controller employing digital circuitry.
Another significant contributor to a complexity of digital processing and the related computational delays in the digital processing of control signals is the signal processing necessary to produce a duty cycle for the switches of the power converter after the input signals are converted into a digital format. This process is frequently performed with a microprocessor or a digital signal processor. Again, either circuit complexity with attendant cost for high performance circuitry or the computational delays of lower performance circuitry is a consequence of substituting digital processing for otherwise conventional analog processes.
The use of controllers employing digital circuitry in power supplies has been the subject of many references including U.S. Pat. No. 6,005,377 entitled “Programmable Digital Controller for Switch Mode Power Conversion and Power Supply Employing the Same,” to Chen, et al. (“Chen”), issued Dec. 21, 1999, which is incorporated herein by reference. Chen discloses a programmable controller that operates in a digital domain without reliance on operational software or internal analog circuitry to control a switch of a power converter. In an exemplary embodiment, the controller is embodied in a field programmable gate array with the ability to handle numerous functions simultaneously and in parallel, as opposed to a digital signal processor which handles instructions serially. Thus, the controllers of Chen can handle bandwidths greater than or equivalent to analog controllers in the range of five to 100 kilohertz. (Column 2, lines 43–57).
Even in view of Chen and other references, the increased switching frequency of the power converter with a continual improvement in circuit technology and the need to maintain a regulated output characteristic is a challenge for the application of controllers employing digital circuitry. The combination of increasing the switching frequency and maintaining a well regulated output characteristic often necessitates the use of higher frequency oscillators within the power converter. The higher frequency oscillators provide clock signals to drive, for instance, the digital-to-analog conversion process employable to translate a digital word representing a duty cycle of the power converter into a time-based signal that can control the switching characteristic of the switches thereof.
For example, if a power converter with a switching frequency of five megahertz is well suited to regulate an output characteristic within a one percent margin of error, then a maximum time resolution for the duty cycle can be estimated as the product of 200 nanoseconds (which is the switching period equivalent to five megahertz), times 50 percent (which is an assumed nominal duty cycle for the switches of the power converter), times one percent (which is the margin of error). The resulting product is one nanosecond, which represents a period for a one gigahertz oscillator that would be necessary to generate clock signals to enable the digital-to-analog conversion process referred to above. One gigahertz oscillators are difficult to implement with ordinary silicon components. In addition, higher frequency oscillators (such as a one gigahertz oscillator) generate substantial heat related to parasitic power loss in the course of rapidly charging and discharging parasitic circuit capacitance as well as incurring switching losses associated with the active elements in the power converter. Power converters operable with oscillator frequencies that are less (e.g., an order of magnitude less) have substantial benefits in cost as well as operating efficiency.
Attempts have been made to make greater utilization of analog circuitry in the digital-to-analog conversion process to avoid the use of an oscillator with the higher frequencies as described above. One alternative is to use an “R-2R” resistor divider to convert the digital word representing duty cycle into an analog format and then use the familiar analog processes employing a periodic ramp and comparator to generate the time-based duty cycle signal. The footprint required for a resistor divider, and a current source and capacitor to generate the periodic ramp have rendered this approach impractical in certain applications such as in systems employing low cost and compact integrated circuits with relatively fine line, digital silicon technology.
As mentioned above, while there has been considerable attention and some improvement in controllers employing digital circuitry (see, for instance, Chen mentioned above) for use with the power converters, there is still an opportunity for improvement in the controllers, especially in view of the more stringent demands on the power converters and the increased switching frequencies thereof. Accordingly, what is needed in the art is a controller for the power converters, and a method of operation thereof, that takes advantage of the benefits associated with digital control circuitry, while overcoming circuit complexities or the need for a higher frequency oscillator (e.g., a one gigahertz oscillator for a power converter with a switching frequency of five megahertz) in the processing of the signals that have disadvantageously affected controllers employing digital circuitry in the past. In accordance therewith, what is needed in the art is a controller that can efficiently produce a duty cycle for the switches of the power converter to maintain an output characteristic at about a desired value without employing a higher frequency oscillator as described above and, at the same time, meet the more exacting demands imposed on the power converters.